A photometer IC can be constructed by using a light-to-voltage sensor or a light-to-current sensor. A light-to-voltage sensor combines a photo-diode 10 and a trans-impedance amplifier 15 on a single monolithic IC, such as the TSL251R light-to-voltage optical sensor (1) described in its data sheet by Texas Advanced Optoelectronic Solutions Inc., and is illustrated in FIG. 1. The trans-impedance amplifier 15 senses the current Iph generated by the photo-diode 10 and outputs a voltage proportional linearly to the photo-generated current. A light-to-current sensor combines a photo-diode 20 and a current amplifier 25 on a single monolithic IC, such as the TPS851 light-to-current optical sensor (2) described in its data sheet by Toshiba Corp., and is illustrated in FIG. 2. Both sensors are widely used to measure the lighting brightness in the displays of cellular phones and portable devices. The trans-impedance amplifier 15 of the light-to-voltage sensor is quite complicated to implement as an integrated circuit, and the current amplifier 25 of the TPS851 light-to-current sensor is implemented in sophisticated bipolar integrated circuits and manufactured using expensive bipolar process technology.
Introductory technical reference for designing the trans-impedance amplifiers and the current amplifiers can be found in the book (3) titled “Analysis and Design of Analog Integrated Circuits” by Paul R. Gray and Robert G. Meyer.
As more functioning chips are packed into electronic portable devices, the demand for smaller and more cost-effective photo-sensor chips increases.